Cell adhesion generally involves recognition of specific adhesive proteins by cell surface receptors. A family of cell surface receptors of particular interest to the present invention are the integrins.
Integrins are a functionally and structurally related group of cell adhesion receptors that interact with a wide variety of ligands including extracellular matrix glycoproteins, complement and other cells. Hynes, Cell, 48:549-554 (1987); Ginsberg et al., Thromb. Haemostas., 59:1-6 (1988); and Ruoslahti et al., Science, 238:491-497 (1987). Integrins participate in cell-matrix and cell-cell adhesion in many physiologically important processes including embryological development, hemostasis, thrombosis, wound healing, phagocytosis, immune and nonimmune defense mechanisms and oncogenic transformation. Two human genetic diseases, Glanzmann'thrombasthenia and leukocyte adhesion deficiency, involve members of the integrin family.
Because of the vital role integrins play in cellular processes, the regulation of integrin function has important biological consequences. One approach to regulating integrin function is by modulating the affinity of the integrin for its ligand. Some members of the integrin family have been shown to require activation to manifest full competence to bind their ligand. See, for example, Wright et al., J. Immunol., 136:1759-1764 (1986) and Detmers et al., J. Cell Biol., 105:1137-1145 (1987) for complement receptor; and see Bennett et al., J. Clin. Invest., 64:1393-1401 (1979) and Marguerie et al., J. Biol. Chem., 254:5357-5363 (1979) for platelet receptor (GPIIb-IIIa). Modulation of the affinity of an integrin for its ligand is the approach taken by the present invention to regulate integrin function.
Structurally, integrins are heterodimeric complexes comprised of noncovalently associated alpha and beta subunits. Within the integrin family there are recognized groups related by the presence of a similar beta subunit and members within each group are distinguished by unique alpha subunits.
For instance, GPIIb-IIIa is a noncovalent, Ca.sup.++ dependent, heterodimer complex comprised of alpha and beta subunits. Jennings et al., J. Biol. Chem., 257:10458-10466 (1982). The alpha subunit, GPIIb consists of a heavy chain (hGPIIb) having a relative molecular weight of about 120 kilodaltons (KDa), and a light chain (lGPIIb) of about 20 KDa that are linked together by disulfide bonds. The beta subunit, GPIIIa is a single chain polypeptide of about 100 KDa. Phillips et al., J. Biol. Chem., 252:2121-2126 (1977). Cell surface molecules immunologically related to GPIIb-IIIa have been identified on a variety of cell types. See Thiagarajan et al., J. Clin. Invest., 75:896-901 (1985); Plow et al., Proc. Natl. Acad. Sci. USA, 83:6002-6006 (1986); and Fitzgerald et al., J. Biol. Chem., 260:10893-10896 (1985).
GPIIb-IIIa contributes to platelet function through interactions with RGD-containing proteins such as fibrinogen [Bennett et al., Proc. Natl. Acad. Sci. USA, 80:2417-2421 (1983)], fibronectin [Ginsberg et al., J. Clin. Invest., 71:619-624 (1983)], and von Willebrand factor [Ruggeri et al., Proc. Natl. Acad. Sci. USA, 79:6038-6041 (1982)], and therefore is a component of the common platelet adhesive protein receptor [Pytela et al., Science, 231:1559-1562 (1986) and Plow et al., J. Biol. Chem., 259:5388-5391 (1984)].
Recent evidence indicates that GPIIb-IIIa is one of several adhesion receptors that share a similar beta subunit and the functional property of recognizing the tripeptide amino acid residue sequence Arg-Gly-Asp (using single letter symbols, RGD). Pytela et al., Science, 231:1559-1562 (1986) and Ruoslahti et al., Cell, 44:517-518 (1986). In addition to GPIIb-IIIa, this group of related receptors includes the vitronectin receptor (VnR) and fibronectin receptor (FnR) isolated from osteosarcoma cells [Pytela et al., Cell, 40:191-198 (1985), Pytela et al., Proc. Natl. Acad. Sci. USA, 82:5766-5770 (1985) and Sanchez-Madrid et al., J. Exp. Med., 158:1785-1803 (1983)].
The similar functional, structural, and antigenic properties of these proteins suggests GPIIb-IIIa and VnR (a .beta..sub.3 type GPIIIa-containing receptor) are members of an adhesion receptor group for which the designation "cytoadhesin" has been proposed. Plow et al., Proc. Natl. Acad. Sci. USA, 83:6002-6006 (1986). Within the cytoadhesin group, distinct alpha subunits combine with a common or very similar beta subunit, resulting in functionally distinguishable receptors. Ginsberg et al., J. Biol. Chem., 262:5437-5440 (1987).
At least two other groups of heterodimeric adhesion receptors have been identified in which a common beta subunit combines with a number of distinct alpha subunits. One group is found on leukocytes and has been referred to as the leukocyte adhesion family and includes LFA-1, Mac-1, and p150,95. Sanchez-Madrid et al., J. Exp. Med., 158:1785-1803 (1983) and Springer et al., Ciba. Found. Symp., 118:102-126 (1986). The other is more widely distributed and has been referred to as the VLA family Hemler et al., J. Biol. Chem., 262:3300-3309 (1987). The beta subunit of the VLA family [Hemler et al., J. Biol. Chem., 262:3300-3309 (1987)]in the chicken has been cloned and sequenced and designated "Integrin" [Tamkun et al., Cell, 46:271-282 (1986)]. The sequence of chicken integrin is similar to that of GPIIIa [Fitzgerald et al., J. Biol. Chem., 262:3936-3939 (1987)]and to the beta subunit of the leukocyte adhesion family [Kishimoto et al., Cell, 48:681-690 (1987)]. Moreover, partial sequences of several alpha subunits also indicate similarities. Ginsberg et al., J. Biol. Chem., 262:5437-5440 (1987); Suzuki et al., Proc. Natl. Acad. Sci. USA, 83:8614-8618 (1986); and Charo et al., Proc. Natl. Acad. Sci. USA, 83:8351-8356 (1986).
The sites on GPIIb-IIIa, or the other cytoadhesins, that are required for their functions as adhesion receptors are not well characterized. Several observations suggest that a functionally significant site on GPIIb-IIIa is near the epitope defined by the monoclonal antibody PMI-1. This antibody binds to the heavy chain of GPIIb [Shadle et al., J. Cell. Biol., 99:2056-2060 (1984)]and defines a region of GPIIb that is associated with several distinct functional activities. First, PMI-1 inhibits adhesion of washed platelets to collagen. Shadle et al., J. Cell. Biol., 99:2056-2060 (1984). Second, the surface orientation of this region is regulated by divalent cations because millimolar (mM) concentrations of calcium or magnesium suppress expression of the PMI-1 epitope. Ginsberg et al., J. Clin. Invest., 78:1103-1111 (1986). Third, abnormal divalent cation regulation of the conformation of this site is associated with a functional thrombasthenic state. Ginsberg et al., J. Clin. Invest., 78:1103-1111 (1986). Fourth, stimulation of platelets with up to 100 micromolar adenosine diphosphate (ADP) or epinephrine, 1 unit per milliliter thrombin, or 50 micrograms per milliliter calf skin collagen does not substantially increase the binding of PMI-1 antibodies to platelets.
Chemical cross-linking studies have localized the ligand-binding site of GPIIb-IIIa to the alpha subunit at a location proximal to the calcium binding site. D'Souza et al., J. Biol. Chem., 265:3440 (1990).
In addition, in the case of certain integrins such as GPIIb-IIIa, the binding of ligand, such as fibrinogen, alters the shape of the receptor resulting in the expression of new antibody-binding sites referred to as ligand-induced binding sites (LIBS). See for example, Frelinger et al., J. Biol. Chem., 263:12397-12402 (1988) and Frelinger et al., J. Biol. Chem., 265:6346-6352 (1990). LIBS were also detected on the vitronectin receptor, suggesting that this property is common to all integrins [Ginsberg et al., J. Biol. Chem., 262:5437-5440 (1987)].
Platelet activation has been reported to produce the appearance of antigenic sites on the platelet surface that are not present in the non-activated platelet, and at least one of such induced sites has been localized to the GPIIb-IIIa receptor complex. Shattil et al, J. Biol. Chem. 260:11107-11114 (1985); Coller, B. S., J. Cell Biol., 103:451-456 (1986).